Wavy membranes and the growth rate of a planar chemical garden: Enhanced diffusion and bioenergetics

Abstract

To model ion transport across protocell membranes in Hadean hydrothermal vents, we consider both theoretically and experimentally the planar growth of a precipitate membrane formed at the interface between two parallel fluid streams in a 2D microfluidic reactor. The growth rate of the precipitate is found to be proportional to the square root of time, which is characteristic of diffusive transport. However, the dependence of the growth rate on the concentrations of hydroxide and metal ions is approximately linear and quadratic, respectively. We show that such a difference in ionic transport dynamics arises from the enhanced transport of metal ions across a thin gel layer present at the surface of the precipitate. The fluctuations in transverse velocity in this wavy porous gel layer allow an enhanced transport of the cation, so that the effective diffusivity is about one order of magnitude higher than that expected from molecular diffusion alone. Our theoretical predictions are in excellent agreement with our laboratory measurements of the growth of a manganese hydroxide membrane in a microfluidic channel, and this enhanced transport is thought to have been needed to account for the bioenergetics of the first single-celled organisms.

Materials Synthesis and Catalysis in Microfluidic Devices: Prebiotic Chemistry in Mineral Membranes

“…In lieu of performing the precipitation in well‐stirred solutions, the parallel flow of reactant solutions in microfluidic devices causes the formation of a nearly linear precipitate membrane at the solution interface. For the example of MnCl 2 and NaOH (Figure 6a), Ding et al . observed a gel‐like layer with large pores on the side close to the Mn 2+ solution.…”

“…For equal reaction times, different concentrations of the reactants yield precipitate membranes of noticeably different thickness (Figure b). Ding et al . reported an analytical model that captures key aspects of the growth dynamics in this experimental situation.…”

Self-assembling iron oxyhydroxide/oxide tubular structures: laboratory-grown and field examples from Rio Tinto

“…The diameter of the incipient tubules is likely to be this finger diameter. With time, diffusion of chemicals in the radial direction supports precipitation, increasing the tubule wall thickness [42]. If tubules grow to have an approximate length L, then the pressure needed to move the fluid through the tubule with diameter d is of order of ρ = 32 µvL/d 2 .…”

Steep pH Gradients and Directed Colloid Transport in a Microfluidic Alkaline Hydrothermal Pore

“…Later, the barrier thickens and ag ranular structure develops at the ocean-facing side.T his behavior suggests the existence of ac ompositional gradient across the barrier in accordance with the literature,which provides detailed structural analyses of similar precipitates. [5,[16][17][18][19] All of the experiments were performed under anoxic conditions to simulate the situation on the early Earth. Thus,t he precipitates are expected to mainly include FeSa nd Fe(OH) 2 .B oth salts have rich chemistries that involve multiple transformations into each other as well as other salts,such as Fe 3 S 4 ,Fe 3 O 4 ,and FeS 2 ,with significant consequences for geochemistry and geochemically powered catalysis.…”